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I just picked up four of the russian 6N7S tubes with the aspiration of puttng together some type of class A (AB-leaning-heavily-A at least) push-pull amp. I chose 6N7S's because they are readily available and they put out a decent amount of power. I was a bit surprised to see that they've been traditionally used for pure class B, but I hope they have some real hi-fi potential.
If each bottle has a 12W max dissipation, I'd dissipate around 20W per pair, hoping for around 10W of nice audio. Is this feasible? I don't have a printer so I haven't done any load line drawing, but let me make sure: with the PPP setup, the load impedance that EACH triode sees is the same as the anode to anode impedance of the OPT right? Any ideas about what OPT primary impedances might be best for such a project?
I'd have to look at some load-lines to see how much swing is needed to drive the tubes but I have a few 5687's kicking around that hopefully could be up to the task. Maybe an LTP feeding a regular grounded-cathode stage? a LTP'ed SRPP?
If such an amp isn't practical I'll just save these to use as somewhat beefy drivers for another future amp. Planning ahead, I am 🙂
In actually looking at the plate curves (something I recommend doing before buying tubes 🙂 ) I saw that this tube probably wants to be biased positive for class A2 operation. I'm looking at around +15-20V bias. what kind of driver do you think could provide all the grid current? maybe some kind of 5687 in a long-tailed SRPP? those are beefy little tubes.
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Hello Soren,
Are we looking at different data? The data sheet I see makes it out to be like a 6N8S with more gain. If so, the 5687s might make better output tubes (and could be driven by a 6N7S diff amp). Mu=35 and Rp=11k would be pretty useful. And 10W total plate dissipation is serious business. See page 3 (link).
Are we looking at different data? The data sheet I see makes it out to be like a 6N8S with more gain. If so, the 5687s might make better output tubes (and could be driven by a 6N7S diff amp). Mu=35 and Rp=11k would be pretty useful. And 10W total plate dissipation is serious business. See page 3 (link).
From what I understand, staying in purely class A with no grid current, the tube behaves like a 6N8S with more gain as you say, but the difference with the 6N7S is that it can be driven positively at much higher currents. Take a look at the plate curves on the bottom left of the second page of the PDF you linked me...
I have to sound unenthusiatic here - I've always hated the 6N7. If you are thinking of PPP then there are much better sounding tubes like the 12b4 and in particular the 1626. Even 6AH4 and 6S4 are nice. All pretty cheap.
my question to you is in what context you've used the 6N7. have you used it with negative bias in its traditional position or have you also tried using it as an output tube? what was the rest of the circuit like? kleep in mind that the 6N7S is Russian-manufacture and might well sound very different from the 6N7.andyjevans said:
some other things: none of the tubes you mentioned has an amount of power equivalent to that of one 6N7S' in parallel. the only reason it's "PPP" is that each bottle has two triodes inside - i'd rather not eat up real estate with 4 separate output tubes per channel.
Same power levels are attainable with a pair of 6CK4 in conventional PP, and linearity is pretty good. (Regarding the objection to four output tubes.)
IMHO Soviet era clones of some Western tubes in my limited experience generally sound somewhat inferior to their Western counterparts, in particular I am referring the various clones of 6SN7, 6SL7 and 6N7.
I haven't used the 6N7 in any recent designs and in the past used them mainly for phase splitting where I ultimately came to the conclusion that other types sounded better.
I have some 6N7S but so far haven't tried anything with them. The experiment you propose would probably be interesting, and if not gratifying other solutions are just a rebuild away.
(Disclaimer: This is only my opinion, is by no means definitive, and also this comment should not be construed as applying to current production. YMMV)
IMHO Soviet era clones of some Western tubes in my limited experience generally sound somewhat inferior to their Western counterparts, in particular I am referring the various clones of 6SN7, 6SL7 and 6N7.
I haven't used the 6N7 in any recent designs and in the past used them mainly for phase splitting where I ultimately came to the conclusion that other types sounded better.
I have some 6N7S but so far haven't tried anything with them. The experiment you propose would probably be interesting, and if not gratifying other solutions are just a rebuild away.
(Disclaimer: This is only my opinion, is by no means definitive, and also this comment should not be construed as applying to current production. YMMV)
Thanks for the advice! Those 6CK4's, in retrospect, would probably have been the way to go, but they are a little pricier and a little scarcer. I see them being used with an OPT of 8K a-a so hopefully if these 6N7S' take the same impedance, I could try switching them in if I find quads or octets or whatever.
Now, for the rest of this prospective amp. I have a couple 6AY3 TV damper diodes I could use as rectifiers, they should be able to handle the current drawn by this amp. I still haven't decided on a splitter/driver setup but as PakProtector suggests over at AudioKarma, an interstage transformer might help with feeding grid current. I've never worked in the A2 range so this should be a learning experience 🙂 Am I right in guessing that if I want to bias at +15V, say, I could use a regulated supply working off of a doubler-rectified 6.3V heater winding? Morgan Jones, IIRC, has a nice schematic for a pot-adjustable regulated bias supply, which I could hopefully translate for positive-bias use..
Now, for the rest of this prospective amp. I have a couple 6AY3 TV damper diodes I could use as rectifiers, they should be able to handle the current drawn by this amp. I still haven't decided on a splitter/driver setup but as PakProtector suggests over at AudioKarma, an interstage transformer might help with feeding grid current. I've never worked in the A2 range so this should be a learning experience 🙂 Am I right in guessing that if I want to bias at +15V, say, I could use a regulated supply working off of a doubler-rectified 6.3V heater winding? Morgan Jones, IIRC, has a nice schematic for a pot-adjustable regulated bias supply, which I could hopefully translate for positive-bias use..
AFter reviewing the plate curves and data..... It looks pretty straight forward..
The curves are for "SINGLE" triode.....and each tube contains 2 triodes...
So I choose to operate the tube plates at 250V .....
Bias it to 0 volts for Class AB2 .... and your quiescent is safe at 15mA which is 3.75W plate dissipation......
You would use 8K plate load for a Push-Pull pair, being a single tube...This would be 10 Watts...
If you use 2 tubes, essentially, P-P-P, you would then parallel the two plates of each tube...then you would load it with a 4K plate load and obtain 20 Watts....
In reality you would want the 250V on the plate at "full output power" .... So this is AB amp, and you most likely will not have a regulated supply... So aim for 280V to 300V at quescent, depending on the supply impedance ....this way when it droops it will be roughly at 250V when fully driven....
For the transformer..... The plate resistance per tube roughly averages 10K ....so you have it internally paralled, this brings it to 5K equivent, then you are in P-P so your source is in series so your back to 10K for your source resistance... This means to get good frequency response from a 4K plate load... (-3dB points at 3Hz and 60kHz) then you would need 136 Henries of inductance and your leakage should be no greater than 37mH ......
These figures are easy to achieve and you should be able to excee them...
Chris
The curves are for "SINGLE" triode.....and each tube contains 2 triodes...
So I choose to operate the tube plates at 250V .....
Bias it to 0 volts for Class AB2 .... and your quiescent is safe at 15mA which is 3.75W plate dissipation......
You would use 8K plate load for a Push-Pull pair, being a single tube...This would be 10 Watts...
If you use 2 tubes, essentially, P-P-P, you would then parallel the two plates of each tube...then you would load it with a 4K plate load and obtain 20 Watts....
In reality you would want the 250V on the plate at "full output power" .... So this is AB amp, and you most likely will not have a regulated supply... So aim for 280V to 300V at quescent, depending on the supply impedance ....this way when it droops it will be roughly at 250V when fully driven....
For the transformer..... The plate resistance per tube roughly averages 10K ....so you have it internally paralled, this brings it to 5K equivent, then you are in P-P so your source is in series so your back to 10K for your source resistance... This means to get good frequency response from a 4K plate load... (-3dB points at 3Hz and 60kHz) then you would need 136 Henries of inductance and your leakage should be no greater than 37mH ......
These figures are easy to achieve and you should be able to excee them...
Chris
Thanks for all the calculation legwork! My only concern is that 10W of audio power coming from a single tube in PP - it's the power output that I'd seen listed for one tube in full class B. I'm shooting for pure class A from a PPP arrangement, and was predicting around 10W from that. If one tube in class B gives 10W, then two tubes in parallel should give 20W - shouldn't class AB be somewhere in between those figures? This is pure armchair logic, which I probably won't have to figure out until later.
For now, though, have I understood the following right?
"an OPT with impedance of 5K will work with PPP 6N7S'"
Does anyone happen to know of any schematics with positive bias off-hand? I'd like to research the implementation. If I get access to a scanner I'll try to sketch something out, but being on a mac I'm having problems figuring out how to run PSUDII and TubeCAD, at least until Leopard comes out.
For now, though, have I understood the following right?
"an OPT with impedance of 5K will work with PPP 6N7S'"
Does anyone happen to know of any schematics with positive bias off-hand? I'd like to research the implementation. If I get access to a scanner I'll try to sketch something out, but being on a mac I'm having problems figuring out how to run PSUDII and TubeCAD, at least until Leopard comes out.
The power output depeneds on the plate load choosen as well as the operating points.... The Class of operation is not the only thing that needs to be looked at...
So you would like this to be Class A P-P-P ????
If so i can do so other number crunching.....
Also....if you already have a specific transformer alreay in hand, is that what you ment by 5K????
If you have a transformer already, then i can fit it to the curves around a optimum operating conditions...
Chris
So you would like this to be Class A P-P-P ????
If so i can do so other number crunching.....
Also....if you already have a specific transformer alreay in hand, is that what you ment by 5K????
If you have a transformer already, then i can fit it to the curves around a optimum operating conditions...
Chris
Hello!
I think that the "Ia, Igt = f( Ugt )" characteristics in the datasheet are simply wrong. They could be, for example, curves for 6N8S... I've seen this type of errors before. For example, this 6N8S datasheet gives Ri(Ia) graph that gives Ri below 500 Ohm at 10 mA...
You might like to have a look at this data. It provides measurements in addition to the official data for 6N7S.
I don't see a decent way to get power out of 6N7S without significant grid currents. However, there is something I am really not sure about...
Triodes are well known to give less distortion with increase of the (Ra/Ri) ratio. However, a triode operated with positive grid bias has "pentode" output characteristics. Certainly, pentodes have pretty different (Ra/Ri) ration for reduced distortion, often 0,1...0,2. So I am not sure what is the optimum plate load for 6N7S operated with positive grid bias. Indeed, in the RCA 6N7 datasheet the plate-to-plate load is given 10 kOhm, and I suppose there is a good reason for that. But this is PP class B amplifier...
One more thing - I am not sure if matching not just tubes, but even triodes in the same bulb will not be needed to get the best sound out of paralleled 6N7S systems. Sometimes the SU products have tolerances enough to affect the sound quality in this case...
Best regards,
Andrey
I think that the "Ia, Igt = f( Ugt )" characteristics in the datasheet are simply wrong. They could be, for example, curves for 6N8S... I've seen this type of errors before. For example, this 6N8S datasheet gives Ri(Ia) graph that gives Ri below 500 Ohm at 10 mA...
You might like to have a look at this data. It provides measurements in addition to the official data for 6N7S.
I don't see a decent way to get power out of 6N7S without significant grid currents. However, there is something I am really not sure about...
Triodes are well known to give less distortion with increase of the (Ra/Ri) ratio. However, a triode operated with positive grid bias has "pentode" output characteristics. Certainly, pentodes have pretty different (Ra/Ri) ration for reduced distortion, often 0,1...0,2. So I am not sure what is the optimum plate load for 6N7S operated with positive grid bias. Indeed, in the RCA 6N7 datasheet the plate-to-plate load is given 10 kOhm, and I suppose there is a good reason for that. But this is PP class B amplifier...
One more thing - I am not sure if matching not just tubes, but even triodes in the same bulb will not be needed to get the best sound out of paralleled 6N7S systems. Sometimes the SU products have tolerances enough to affect the sound quality in this case...
Best regards,
Andrey
Andro_BG said:
I looked at the Anode Current, Ia, The graph matches this spec...
Which states 300V , 0V bias : as 17mA with a +/- 5mA tolerance...which lines up pretty close....
Chris
Edit: I apologize, I was wrong. Sorry - and thank you for pointing me my mistake! I have considered 6N7S too "closed" 🙂
Best regards,
Andrey
Best regards,
Andrey
It is possible to design the entire amp circuit to use the 6N7S tubes..... It can be a Class A input valve as well as used for the phase inverter and the output stage...
Would you be interested in something like that????
Chris
Would you be interested in something like that????
Chris
I really would - it makes sense, but at the moment I only actually have four of the tube coming, so I'd probably need to get four more. It'd be AMAZING to be able to sub out between that many tubes to bias perfectly, and plus, having a row of eight of the exact same tube would also be mindblowing. I don't have one specific OPT lying around; I just wanted to get an idea of what I could consider using. Is there a range of acceptable primary impedances that could work with PPP 6N7S'?
Here are some decisions to make....
If you want a strictly Class A2 amp then the best you can sqeeze out of this is 4 Watts in Push-Pull with a 3.2K Plate-Load operating at 120V on the plate with a +15V bias.....
This is pushing the poor tube to it's limits and probably won't have good long life.... SO if you use P-P-P with a 1.6K plate load then you get 8 Watts..
For a Class AB2 amp that is a healthy design, then you would operate the Push-Pull circuit at 200V and bias around 20mA to 25mA per triode, with a 6K plate load. This would give you 7.5W ....so if you go P-P-P then you are at 15W with a 3K plate load... SInce each tube has two triodes internally that you would connect in parallel, then you only use 2 tubes for P-P-P ... This would leave you 2 remaining tubes...
It is possible to use another one of these tubes as a input tube with gain and phase inversion all in one... Long-Tailed Pair with just a tiny bit of feed-back from secondary of OPT to the bottom tail resistor....
So a total of 3 of these tubes would make an entire amplifier...
Another AB2 choice is running 0V on the grid with the plates at 250V...this biases it at roughly 15mA per triode...
Then if you use a 10K plate load you get 10Watts ....
So P-P-P would be 20W into a 5K plate load.... This would be your most efficient choice...
Chris
If you want a strictly Class A2 amp then the best you can sqeeze out of this is 4 Watts in Push-Pull with a 3.2K Plate-Load operating at 120V on the plate with a +15V bias.....
This is pushing the poor tube to it's limits and probably won't have good long life.... SO if you use P-P-P with a 1.6K plate load then you get 8 Watts..
For a Class AB2 amp that is a healthy design, then you would operate the Push-Pull circuit at 200V and bias around 20mA to 25mA per triode, with a 6K plate load. This would give you 7.5W ....so if you go P-P-P then you are at 15W with a 3K plate load... SInce each tube has two triodes internally that you would connect in parallel, then you only use 2 tubes for P-P-P ... This would leave you 2 remaining tubes...
It is possible to use another one of these tubes as a input tube with gain and phase inversion all in one... Long-Tailed Pair with just a tiny bit of feed-back from secondary of OPT to the bottom tail resistor....
So a total of 3 of these tubes would make an entire amplifier...
Another AB2 choice is running 0V on the grid with the plates at 250V...this biases it at roughly 15mA per triode...
Then if you use a 10K plate load you get 10Watts ....
So P-P-P would be 20W into a 5K plate load.... This would be your most efficient choice...
Chris
sorry for digging up an old thread, I was studying the 6n7 datasheet and it states pretty clearly that in class A, plates should see a Pdmax of 1.0w
my question is, how did we come to the conclusion that we could dump anything more than 1.0w on the plates without having it glow?
the 5.5w rating per triode is clearly for class B operation only right? A continuous 5.5w Pdiss on the plates (200v @ 25ma) will certainly wreck the tubes?
i have a pair of 6a6 on the way in and am a bit worried that I am expecting too much, I'm shooting for 160v @ 50ma, Vg = +15v per triode in just PP, not PPP.
if my math serves me well, if I swing pure class A2 from +15 to +30 and back to 0v, I should get about 12w on a 5K PP output trans. i shudder to think that I might just set the plates a light.
cheers guys!
my question is, how did we come to the conclusion that we could dump anything more than 1.0w on the plates without having it glow?
the 5.5w rating per triode is clearly for class B operation only right? A continuous 5.5w Pdiss on the plates (200v @ 25ma) will certainly wreck the tubes?
i have a pair of 6a6 on the way in and am a bit worried that I am expecting too much, I'm shooting for 160v @ 50ma, Vg = +15v per triode in just PP, not PPP.
if my math serves me well, if I swing pure class A2 from +15 to +30 and back to 0v, I should get about 12w on a 5K PP output trans. i shudder to think that I might just set the plates a light.
cheers guys!
